1. FIELD OF THE INVENTION
This invention relates to a method of manufacturing integrally formed plastic sandwiches. More particularly, the invention relates to a process for the extrusion of integral plastic members having solid skins and a cellular inner portion.
2. DESCRIPTION OF THE PRIOR ART
Attempts have been made in the past to provide for the production of plastic sandwich structural elements by bonding solid plastic skin members to the top and bottom surfaces of a cellular plastic core. However, this method of fabrication is costly since it requires the individual fabrication of the individual skin and core members, the application of a relatively costly adhesive material and the clamping of the bonded structural element until the adhesive is set. In addition, the difficulty in obtaining a high strength bond over an extended surface using an adhesive results in a product having a less than desirable level of reliability.
Other methods for the fabrication of plastic sandwich members have in the past included a molding process in which inert gas is dispersed in a polymer melt, the polymer melt is introduced into a mold and as the mold fills, the gas expands within the material forming a cellular core while a solid plastic skin forms along the walls of the mold. This method has the disadvantages of not permitting close control over the skin thickness, not permitting variations in material between the skin and the core and requiring a large and consequently expensive mold for theproduction of large plastic members.
Attempts have been made to co-extrude a multi-layer plastic sandwich such as disclosed in the Patent to Chisholm et al U.S. Pat. No. 3,557,256. This patent no where discloses the concept of providing a foamed core. In Thomas, U.S. Pat. No. 3,331,900 there is disclosed a skin coated foamed plastic article. Shippers, U.S. Pat. No. 3,782,870 disclosed an apparatus for partially foaming a portion of extruded material by separating melt streams and then rejoining the separate melt streams and partially foaming one of the melt streams.